The F-T process for converting natural gas or other gaseous fossil fuels to higher liquid hydrocarbons, well known to those of ordinary skill in the art, requires a synthesis gas of primarily CO and H2. This synthesis gas is typically generated in a steam reformer, auto-thermal reformer (ATR), or the like. ATR involves the reforming of O2, steam, and methane to produce CO and H2. The synthesis gas may also be generated by partially oxidizing natural gas with oxygen. This natural gas may contain some higher or heavy hydrocarbons along with CO2 and N2.
The synthesis gas fed to the F-T process is typically high in CO and H2 after condensing the excess water used in synthesis gas generation. The water vapor is typically near zero at the high pressures (approximately 25 bar) used in the F-T reactor. The residual methane is typically around 1%, which indicates that the synthesis gas generation was efficient and did not allow significant methane slip. The combination of CO2, N2, and CH4 are considered inert in the F-T reaction and, as a consequence, there is not a hard maximum specification. However, the presence of these inerts requires larger reactors and higher total pressures. It is, therefore, important to minimize the inerts, especially the CO2 and CH4, which may be controlled or removed, while the N2 is somewhat fixed by the nitrogen in the feed gas (i.e. natural gas).
The F-T reaction converts CO+2H2 to —(CH2)-+H2O. The —(CH2)- forms a chain and acts as a building block for the liquid hydrocarbons. Also exiting the F-T reactor is a tail gas that contains CO, H2, CO2, H2O, N2, CH4, and some heavy hydrocarbons and olefins. Part of the tail gas is typically recycled back to the F-T reactor, with the remainder used as fuel in the plant. Since the tail gas contains significant amounts of CO and H2, it does not make sense to recycle the tail gas back to the ATR, where CO and H2 are first partially oxidized with oxygen to CO2 and H2O.
Burning the tail gas in the plant is a way to prevent nitrogen buildup in the F-T recirculating loop by venting an amount of nitrogen that is equal to the incoming nitrogen in the natural gas and nitrogen associated with the ATR oxygen. However, combusting more than that required for nitrogen elimination is a waste of CO and H2 made in the synthesis gas generation step. This means that either the synthesis gas generator has to be larger, along with the oxygen plant, to make enough CO and H2 for the F-T reactor or that the F-T reactor will make less hydrocarbon liquid.
U.S. Pat. No. 6,696,501 (Schanke et al., Feb. 24, 2004) provides:                A method is described for conversion of natural gas or other fossil fuels to higher hydrocarbons, comprising the following steps: a) reaction of natural gas with steam and oxygenic gas in at least one reforming zone in order to produce a synthesis gas consisting primarily of hydrogen and CO, in addition to some carbon dioxide; b) passing said synthesis gas to a Fischer-Tropsch reactor in order to produce a crude synthesis stream consisting of lower hydrocarbons, water and non-converted synthesis gas; c) separation of said crude synthesis stream in a recovery zone, into a crude product stream mainly containing heavier hydrocarbons, a water stream and a tail gas stream mainly containing the remaining constituents; which is characterized in that the method also comprises the following steps; d) stream reformation of at least part of the tail gas in a separate steam reformer; e) introduction of the reformed tail gas into the gas stream before this is led into the Fischer-Tropsch reactor.        
Thus, U.S. Pat. No. 6,696,501 proposes steam reforming the F-T tail gas plus additional natural gas in order to increase carbon efficiency and lower the oxygen consumption of the ATR by reducing the amount of feed gas to the ATR. U.S. Pat. No. 6,696,501 proposes steam reforming at typical steam reforming conditions of 10 to 40 bar (i.e. at high pressures) and a temperature of 850 to 950 degrees C. U.S. Pat. No. 6,696,501 assumes various steam to carbon and CO2 to carbon ratios of 5.3, 1.0, and 0.6. A steam reforming catalyst supplier lists typical steam to carbon ratios of 2.5 to 5.0 for reforming pressures of 15 to 35 bar. Equilibrium calculations with a typical F-T tail gas and the various conditions provided in the patent demonstrate that at steam to carbon and CO2 to carbon ratios of 5.0, the CO2 in the reformed gas is 27% at 10 bar. The calculations also demonstrate that at steam to carbon and CO2 to carbon ratios of 1.0, the CO2 and CH4 in the reformed gas are 9% and 4.7%, respectively, at 25 bar. The calculations further demonstrate that operation at steam to carbon and CO2 to carbon ratios of 0.6, carbon soot forms in the reformer at pressures above 15 bar. This is, of course, problematic.